feat: don't sample an index and its blinder index at the same time

tensor_pcs/src/utils.rs

@@ -1,8 +1,6 @@
-use tiny_keccak::{Hasher, Keccak};
-
-use crate::FieldExt;
-
 use crate::transcript::Transcript;
+use crate::FieldExt;
+use tiny_keccak::{Hasher, Keccak};
 
 pub fn rlc_rows<F: FieldExt>(x: Vec<Vec<F>>, r: &[F]) -> Vec<F> {
     debug_assert_eq!(x.len(), r.len());
@@ -68,7 +66,7 @@ pub fn sample_indices(
     let mut indices = Vec::with_capacity(num_indices);
     let mut counter: u32 = 0;
 
-    // TODO: Don't sample at n and n + N
+    let n = max_index / 2;
     while indices.len() < num_indices {
         let mut random_bytes = [0u8; 64];
 
@@ -76,10 +74,8 @@ pub fn sample_indices(
         transcript.challenge_bytes(&mut random_bytes);
 
         let index = sample_index(random_bytes, max_index);
-        if !indices.contains(&index)
-        // || !indices.contains(&(index + (max_index / 2)))
-        // || !indices.contains(&(index - (max_index / 2)))
-        {
+        let pair_index = if index > n { index - n } else { index + n };
+        if !indices.contains(&index) && !indices.contains(&pair_index) {
             indices.push(index);
         }
         counter += 1;
@@ -87,3 +83,27 @@ pub fn sample_indices(
 
     indices
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    type F = halo2curves::secp256k1::Fp;
+
+    #[test]
+    fn test_sample_indices() {
+        let mut transcript = Transcript::<F>::new(b"test_sample_index");
+        let num_indices = 10;
+        let max_index = 100;
+        let indices = sample_indices(num_indices, max_index, &mut transcript);
+
+        assert_eq!(indices.len(), 10);
+        let n = max_index / 2;
+        for index in &indices {
+            if *index > n {
+                assert!(!indices.contains(&(index - n)));
+            } else {
+                assert!(!indices.contains(&(index + n)));
+            }
+        }
+    }
+}